The structure of lignin that we used for production of carbon fibers
were characterized by a set of multiple nuclei NMR spectra,
especially the 2-D HMBC spectrum which reveals the strong
coupling between Hs and adjacent Cs among the linkages and the
phenyl groups, and was therefore used as the main evidence for the
assignment of lignin structure. The 13C NMR spectra of the samples
prior to and after filtration through ceramic membrane are shown in
Fig.1(a) and (b), respectively. It can be found that only the signals at
97.3 (artifacts), 65.8e67.1 (C6 in cellulose) and 23.4e33.3 ppm
(lipids (CH2) and protein (sc)) were removed after filtration while
other resonances remain the same before and after filtration
[21e23]. This means that cellulose, carbohydrates and other impurities
which are not conducive to melt spinning can be effectively
removed by purification. Detailed analysis of the NMR spectra,
especially the 2-D HMBC spectrum, reveals that the purified lignin
contains S and G units together with b-b and b-1 linkages with a
linear molecular structure, as shown in Fig. S1eS3. For example, the
signal at 81.8 ppm was assigned to the Ca in b-b units, while the
peaks at 85.7ppmand 71.4ppmcorresponded to the Ca and Cg in b-1
units, respectively.